Carbon nanotubes (CNT) are promising materials for transparent conduction as a result of their exceptional electrical, optical, mechanical, and chemical properties. Ultra thin films based on CNT networks above the percolation limit have beneficial attributes such as stiffness and chemical stability that makes it superior to indium tin oxide (ITO) in certain applications. CNT nano-mesh films exhibit flexibility, allowing films to be deposited on pliable substrates prone to acute angles, bending, and deformation, without fracturing the coating. Modeling work has shown that CNT films may offer potential advantages such as, for example, tunable electronic properties through chemical treatment and enhanced carrier injection owing to the large surface area and field-enhanced effect at the nanotube tips and surfaces. It is also recognized that although ITO is an n-type conductor, such CNT films can be doped p-type and, as such, can have applications in, for instance, the anode or injecting hole into OLED devices, provided the films are smooth to within 1.5 nm RMS roughness.
Although ITO films still lead CNT films in terms of sheet conductance and transparency, the above-mentioned advantages together with potential cost reductions have stimulated significant interest in exploiting carbon nanotube films as transparent conductive alternative to ITO. In order to live up to its expectations, CNT films should display high transparency coupled with low sheet resistance. The relationship between transparency and sheet resistance for thin conducting films is controlled by the ratio of dc conductivity and optical conductivity, σdc/σopt, such that high values of this ratio typically are most desirable.
However, to date, viable CNT synthetic methods yield poly-dispersed mixtures of tubes of various chiralities, of which roughly one-third are metallic with the remainder being semiconducting. The low ρdc/σopt performance metric of such films is largely related to the large fraction of semiconducting species. These semiconducting tubes, in turn, also give rise to the bundling of the tubes, which tends to increase the junction resistance of the film network.
The typical value of σopt for CNT films depends on the density of the film. Just above the percolation limit, this value tends to close at 1.7×104 S/m at 550 nm, while the dc electrical conductivity to date is in the region of 5×105 S/m. However, industry specifications require better than 90% transmission and less than 90 ohms/square sheet resistance. To achieve these values, one can determine that the necessary dc conductivity be in excess of 7×105 S/m. Thus, it will be appreciated that there is a need in the art for improving the electronic quality of even the best CNT films so that the σdc/σopt ratio, in turn, is improved. This poly-dispersity stems from the unique structure of SWNTs, which also renders their properties highly sensitive to the nanotube diameter.
Certain example embodiments of this invention relate to the deposition of nano-mesh CNT films on glass substrates and, in particular, the development of coatings with high σdc/σopt on thin, low iron or iron free soda lime glass and/or other substrates (e.g., other glass substrates such as other soda lime glass and borosilicate glasss, plastics, polymers, silicon wafers, etc.). In addition, certain example embodiments of this invention relate to (1) finding viable avenues of how to improve the σdc/σopt metric via stable chemical doping and/or alloying of CNT based films, and (2) developing a large area coating technique suitable for glass, as most work date has focused on flexible plastic substrates. Certain example embodiments also pertain to a model that relates the morphological properties of the film to the σdc/σopt.
In certain example embodiments of this invention, a method of making a coated article comprising a substrate supporting a carbon nanotube (CNT) inclusive thin film is provided. A CNT-inclusive ink is provided. Rheological properties of the CNT-inclusive ink are adjusted by adding surfactants to the ink so that any semiconducting CNTs located within the ink are less likely to clump together. The ink having the adjusted rheological properties is applied to the substrate to form an intermediate coating. A material is provided over the intermediate coating to improve adhesion to the substrate. The intermediate coating is doped with a salt and/or super acid so as to chemically functionalize the intermediate coating in forming the CNT-inclusive thin film.
In certain example embodiments of this invention, a method of making a coated article comprising a substrate supporting a carbon nanotube (CNT) inclusive thin film is provided. A CNT-inclusive ink is provided, with the CNT-inclusive ink comprising double-wall nanotubes. Rheological properties of the CNT-inclusive ink are adjusted to make the CNT-inclusive ink more water-like. The ink having the adjusted rheological properties is applied to the substrate to form an intermediate coating using a slot die apparatus. The intermediate coating is dried or allowed to dry. An overcoat is provided over the intermediate coating to improve adhesion to the substrate. The intermediate coating is doped with a salt and/or super acid so as to chemically functionalize the intermediate coating in forming the CNT-inclusive thin film. The CNT-inclusive film is substantially planarized.
In certain example embodiments of this invention, a method of making a coated article comprising a substrate supporting a carbon nanotube (CNT) inclusive thin film is provided. A CNT-inclusive ink is provided. Rheological properties of the CNT-inclusive ink are adjusted by adding surfactants to the ink so that any semiconducting CNTs located within the ink are less likely to clump together. The ink having the adjusted rheological properties is applied to the substrate to form an intermediate coating. A material is provided over the intermediate coating to improve adhesion to the substrate. The intermediate coating is doped with a salt and/or super acid so as to chemically functionalize the intermediate coating in forming the CNT-inclusive thin film. Silver nanowires are formed, directly or indirectly, on the substrate to provide a long-distance charge transport to reduce the number of any carbon nanotube junctions that may be formed thereon.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.